In the past, as an analyzer adapted to measure the concentration of a predetermined component contained in sample gas such as exhaust gas, there has been one using, for example, Fourier transform infrared (FTIR) spectroscopy (Patent Literature 1).
In the analyzer using the FTIR spectroscopy, a pump is provided on the upstream side of a measurement cell, and the pressure of sample gas in the measurement cell is atmospheric pressure or a pressure close to atmospheric pressure. That is, the pressure at the time of sample gas measurement is atmospheric pressure or a pressure close thereto.
In addition, the concentration of a predetermined component is obtained by performing concentration calculation using a calibration curve obtained from the relationship between multiple known gas concentrations and corresponding absorbances and making a correction based on the pressure of the sample gas because the pressure of the sample gas at the time of measurement affects the concentration.
As described above, since the pressure of the sample gas at the time of measurement is atmospheric pressure or a pressure close to atmospheric pressure, as the calibration curve, one prepared at atmospheric pressure is used. In this case, the concentration correction based on the pressure is made using a linear relational expression passing through the origin on the assumption that the relationship between pressure and concentration is a simple proportional relationship. This is because, usually, on the assumption that since the pressure at the time of measurement is close to atmospheric pressure, the effect of pressure broadening is small, and absorbance changes in proportion to the partial pressure of the gas, the correction is made using the linear relational expression having no intercept. Specifically, the pressure correction is made using the following expression.
                              C                      x            ⁢                                                  ⁢            _            ⁢                                                  ⁢            press                          =                              C            x                    ×                                    a              x                        P                                              [                  Expression          ⁢                                          ⁢          1                ]            where Cx_press is the concentration of a component x after the pressure correction, ax is a pressure correction coefficient for the component x (usually ax=P0: reference pressure), P: is the pressure inside a measurement cell [kPa], and Cx is the concentration of the component x before the pressure correction.